In this paper. a SIW three apertures directional coupler is designed by the theory of rectangular waveguide multi-aperture directional coupler and the equivalent relation between SIW and rectangular waveguide. Then, a new three apertures directional coupler is proposed through opening a dumbbell-shaped slot on the metallic surface of coupling aperture. The experiment result shows that the performance of the directional coupler is greatly enhanced. Within the frequency range of 8.6 GHz-10.6 GHz, the reflection coefficient of the input and transmission coefficient of the isolated port are both below -15 dB, and the relative bandwidth is 20%. Transmissions of the straight port and coupled port in the pass-band are between -8.2 dB and -3.3 dB. This coupler's coupling is about 6 dB, and the isolation is over 15 dB.